dotnet/wpf · error · ArgumentException

ArgumentException(SR.Format(SR.MemberHasInvalidXamlName…

Error message

ArgumentException(SR.Format(SR.MemberHasInvalidXamlName, property.Name), nameof(property))

What it means

WriteStartMember validates that the XamlMember's Name is a valid XAML name; if property.IsNameValid is false it throws ArgumentException including the member's name. Member names become XML attribute/element names, so illegal characters are rejected up front.

Solutions

Correct the member's Name to a valid XAML identifier before writing.
Check property.IsNameValid before the call and handle invalid members separately.
Use a properly registered attachable-property definition instead of hand-built member names.

Example fix

// before
writer.WriteStartMember(new XamlMember("My Property", type, false)); // ArgumentException (space)
// after
writer.WriteStartMember(new XamlMember("MyProperty", type, false));

Defensive patterns

Strategy: validation

Validate before calling

if (property is null) throw new ArgumentNullException(nameof(property));
if (!property.IsNameValid) throw new ArgumentException($"Member '{property.Name}' has an invalid XAML name", nameof(property));
writer.WriteStartMember(property);

Type guard

bool IsWritableMember(XamlMember m) => m is not null && m.IsNameValid;

Try / catch

try { writer.WriteStartMember(property); }
catch (ArgumentException ex) { /* fix member name or skip member */ }

Prevention

Check IsNameValid before WriteStartMember for dynamically built members.
Use valid NCName identifiers when defining attachable properties.
Never embed spaces or punctuation in member names by concatenation.

When it happens

Trigger: Calling xamlXmlWriter.WriteStartMember(property) with a XamlMember whose Name contains characters invalid for XAML/XML names (spaces, invalid leading chars, etc.).

Common situations: Manually constructed XamlMember instances with ad-hoc names; attached-property names built by string concatenation; members derived from CLR names that were renamed or generated with characters invalid in XML names.

Understand the failure class

Background: "invalid id" errors: invalid identifier format — why libraries reject IDs before lookup, and how to fix them — this error's family across 37 libraries.

Related errors

AI-assisted analysis of dotnet/wpf@81131a70a4 (2026-09-14). Data as JSON: /api/errors/9aceb24b6edf06ae. Report an issue: GitHub.

Appendix: source

Thrown at src/Microsoft.DotNet.Wpf/src/System.Xaml/System/Xaml/XamlXmlWriter.cs:191

                WriteTypeArguments(type);
            }
        }

        public override void WriteEndObject()
        {
            CheckIsDisposed();
            currentState.WriteEndObject(this);
        }

        public override void WriteStartMember(XamlMember property)
        {
            CheckIsDisposed();

            ArgumentNullException.ThrowIfNull(property);

            if (!property.IsNameValid)
            {
                throw new ArgumentException(SR.Format(SR.MemberHasInvalidXamlName, property.Name), nameof(property));
            }

            currentState.WriteStartMember(this, property);
        }

        public override void WriteEndMember()
        {
            CheckIsDisposed();
            currentState.WriteEndMember(this);
        }

        public override void WriteValue(object value)
        {
            CheckIsDisposed();
            if (value is null)
            {
                WriteStartObject(XamlLanguage.Null);
                WriteEndObject();

View on GitHub (pinned to 81131a70a4)